It took metallurgists one hundred and fifty years after Scheele's discovery of “tungsten” in 1781, to develop and then apply tungsten carbide (“WC”) cermets in industry. Cermets are a composite material consisting of a combination of metallic and ceramic materials. The most common cermets are the cemented carbides, composed of an extremely hard ceramic (e.g., WC, TiC), bonded together by a ductile metal such as cobalt or nickel. Many different types of polycrystalline WC-based powders (e.g., U.S. Pat. Nos. 4,664,899; 5,746,803; 6,007,598) and densified polycrystalline ceramics (e.g., U.S. Pat. Nos. 4,828,584; 5,563,107; 5,612,264; 5,681,783; 6,033,789) have been described.
Today, the main use of tungsten (in the form of WC) is in the manufacture of cemented carbide. Due to the wide application of cemented carbide products in engineering, electronics, mining, manufacturing, aerospace, and medicine, the annual world market demand for cemented carbide now exceeds some 30,000 tons. In manufacturing, for example, durable and effective cutting tools for high-speed machining have significant commercial applications. The material comprising these cutting tools should have a high heat hardness, a high transverse rupture strength, and fracture toughness. Further, these cutting tools are designed to control the flow of “chips” which are formed in the machining process, and, to reduce the cutting forces.
Polycrystalline WC-based cermets have shown two fundamental limitations. Under certain conditions, WC-based cermets lack toughness (i.e., they are brittle in response to mechanical shock), and at the high temperatures caused by faster feed rates, they undergo plastic deformation. Polycrystalline WC is, therefore, disfavored in the preparation of densified ceramic bodies for select applications (e.g., cutting tool inserts with chip control) because WC is not thought to have the needed toughness and wear resistance.
These limitations have also impeded the machining rate of titanium alloys. Specifically, titanium and associated alloys have been used in the aerospace field extensively, due to their superior strength to weight ratio and exceptional corrosion resistance. Although the use of titanium alloy has increased in recent years, machining speeds have remained the same, unlike, for example, high nickel alloy materials, where machine speeds have increased with the advent of silicon nitrides and whisker reinforced ceramics (from 125 sfm to 600-1000 sfm). In contrast, the machining speeds for titanium alloys have remained at around 250 sfm or lower for the past 20 years.
Machining is normally accomplished with cemented tungsten carbide tools (sub micron to 1-5 micrometer WC grain size) with five to six weight percent cobalt made by conventional liquid phase sintering at temperatures near 1400 degree(s) C. in vacuum or hydrogen. Machining speeds for such tools are still between 120 sfm and 300 sfm when machining titanium with depths of cut between 0.020 and 0.050 per revolution. This is very slow compared to machining of other metals and represents a barrier that many tool developers have sought to overcome. Numerous cemented carbides, ceramic-coated cemented carbides, and available ceramics have all been tested with limited success in order to find a cutting tool that can machine titanium alloys at higher speeds than WC-Co tools. The exception to these results is polycrystalline WC, which has shown promise for machining titanium. See U.S. Pat. No. 4,828,582 and J. Kertesz, et al., “Machining Titanium Alloys with Ceramic Tools,” J. Metals, (1988), 50-51.
In general, hardness of the cermets, i.e., wear resistance, strength, and toughness, can be changed by WC grain size, cobalt content, and by other carbides present. In formulating these materials, however, there is a tendency that if wear resistance is heightened, fracture resistance is lowered. Conversely, if fracture resistance is heightened, wear resistance is lowered. Accordingly, in the design of densified polycrystalline WC-containing ceramic bodies, it has been a challenge to improve one material property without adversely affecting another material property by adding cobalt, or another iron group, that will plastically deform in high heat, e.g., high-speed machining.